The Letter of Agatho, Pope of Old Rome, to the Emperor Constantine IV, and the Letter of Agatho and of 125 Bishops of the Roman Synod, Addressed to the Sixth Council:

“For this is the rule of the true faith, which this spiritual mother of your most tranquil empire, the Apostolic Church of Christ, has both in prosperity and in adversity always held and defended with energy; which, it will be proved, by the grace of Almighty God, has never erred from the path of the apostolic tradition, nor has she been depraved by yielding to heretical innovations, but from the beginning she has received the Christian faith from her founders, the princes of the Apostles of Christ, and remains undefiled unto the end, according to the divine promise of the Lord and Saviour himself, which he uttered in the holy Gospels to the prince of his disciples: saying, ‘Peter, Peter, behold, Satan has desired to have you, that he might sift you as wheat; but I have prayed for you, that (your) faith fail not. And when you are converted, strengthen your brethren.’ Let your tranquil Clemency therefore consider, since it is the Lord and Saviour of all, whose faith it is, that promised that Peter’s faith should not fail and exhorted him to strengthen his brethren, how it is known to all that the Apostolic pontiffs, the predecessors of my littleness, have always confidently done this very thing: of whom also our littleness, since I have received this ministry by divine designation, wishes to be the follower, although unequal to them and the least of all.

To these same commissioners we also have given the witness of some of the holy Fathers, whom this Apostolic Church of Christ receives (quos haec apostolica Christi ecclesia suscipit), together with their books, so that, having obtained from the power of your most benign Christianity the privilege of suggesting, they might out of these endeavour to give satisfaction, (when your imperial Meekness shall have so commanded) as to what this Apostolic Church of Christ, their spiritual mother and the mother of your God-sprung empire (quid haec spiritualis mater eorum ac a Deo propagate imperii apostolica Christi ecclesia), believes and preaches, not in words of worldly eloquence… but that they set forth this tradition of the Apostolic See (sed traditionem hujus apostolicae sedis) in all sincerity as it has been taught by the apostolic pontiffs, who were our predecessors.

because the true confession thereof for which Peter was pronounced blessed by the Lord of all things, was revealed by the Father of heaven, for he received from the Redeemer of all himself, by three commendations, the duty of feeding the spiritual sheep of the Church; under whose protecting shield, this Apostolic Church of his has never turned away from the path of truth in any direction of error (hec apostolica ejus ecclesia nunquam a via Veritatis in qualibet erroris parte deslexa est), whose authority, as that of the Prince of all the Apostles, the whole Catholic Church (omnis catholica … ecclesia), and the Ecumenical Synods have faithfully embraced, and followed in all things; and all the venerable Fathers have embraced its Apostolic doctrine, through which they as the most approved luminaries of the Church of Christ have shone; and the holy orthodox doctors have venerated and followed it, while the heretics have pursued it with false criminations and with derogatory hatred.

Therefore the Holy Church of God, the mother of your most Christian power, should be delivered and liberated with all your might (through the help of God) from the errors of such teachers, and the evangelical and apostolic uprightness of the orthodox faith, which has been established upon the firm rock of this Church of blessed Peter, the Prince of the Apostles, which by his grace and guardianship remains free from all error, [that faith I say] the whole number of rulers and priests, of the clergy and of the people, unanimously should confess and preach with us as the true declaration of the Apostolic tradition, in order to please God and to save their own souls.

For this is the rule of the true faith, which this spiritual mother of your most tranquil empire, the Apostolic Church of Christ, has both in prosperity and in adversity always held and defended with energy; which, it will be proved, by the grace of Almighty God, has never erred from the path of the apostolic tradition, nor has she been depraved by yielding to heretical innovations, but from the beginning she has received the Christian faith from her founders, the princes of the Apostles of Christ, and remains undefiled unto the end, according to the divine promise of the Lord and Saviour himself, which he uttered in the holy Gospels to the prince of his disciples: saying, ‘Peter, Peter, behold, Satan has desired to have you, that he might sift you as wheat; but I have prayed for you, that (your) faith fail not. And when you are converted, strengthen your brethren.’ Let your tranquil Clemency therefore consider, since it is the Lord and Saviour of all, whose faith it is, that promised that Peter’s faith should not fail and exhorted him to strengthen his brethren, how it is known to all that the Apostolic pontiffs, the predecessors of my littleness, have always confidently done this very thing: of whom also our littleness, since I have received this ministry by divine designation, wishes to be the follower, although unequal to them and the least of all.

“Resting on Peter’s protection, this Apostolic Church of his has never turned aside from the way of truth to any part of error, and her authority has always been faithfully followed and embraced as that of the prince of the Apostles by the whole Catholic Church and all Councils, and by all the venerable Fathers who embraced her doctrine…..and she [the Roman church], by the grace of almighty God, will be proved never to have wandered from the path of apostolic tradition, nor to have succumbed to the novelties of heretics; but even as in the beginning of the Christian faith she received it from her founders, the princes of the Apostles of Christ, so she remains unspotted to the end, according to the divine promise of our Lord and Savior Himself…which He spoke to the prince of His apostles in the holy Gospels: ‘Peter, Peter, says He, behold Satan has desired to have you, that he might sift you as he who sifts wheat; but I have prayed for thee, that they faith fail not, and thou one day being converted, strengthen thy brethren’. Let your clemency [Emperor Constantine] therefore consider that the Lord and Savior of all, to whom faith belongs, who promised that the faith of Peter should not fail, admonished him to confirm his brethren; and it is known to all men that the Apostolic Pontiffs [of Rome], predecessors of my littleness, have always done this with confidence…” (Letter of Pope St. Agatho to the Byzantine Emperor & Council of Constantinople III – AD 681)